1. Technical Field
The present disclosure relates to a light-emitting device, and more particularly, to an organic light-emitting device.
2. Description of Related Art
In recent years, many researches have been dedicated to electroluminescent device technology, in which an organic light-emitting device (e.g., a light-emitting diode) attracted even more attention. The so-called electroluminescence refers to energy in a form of light wave released by electrons dropping down to lower energy level from higher energy level (excited state).
The electroluminescent structure of the basic organic light-emitting device includes an organic light-emitting material interposed between two electrodes. First, holes and electrons are injected to a light-emitting layer by an anode (typically made of indium tin oxide (ITO) transparent electrode) and a cathode, respectively, and those holes and electrons then combine in the layer, which is made of the light-emitting material, to excite the light-emitting molecules to an excited state from a ground state. When those molecules come back to the ground state from the excited state, energy is released in a form of light; that is, electric energy is transformed into light wave. Briefly, there are electric currents flowing through the light-emitting layer, such that the electric energy enables the light wave from the light-emitting material. However, if the light-emitting layer were made of a single kind of material (100%), it would cause excitons quenching and thus severely decrease luminous efficiency. Currently, co-evaporation of both a host and a dopant (or a dye) is used to form the light-emitting layer in the OLED like a phosphorescent OLED.
Concerning the molecular energy level of the light-emitting layer of the phosphorescent light-emitting diode, when applying voltage, electrons in the reductive state molecules of the host material and holes in the oxidative state molecules thereof are separately injected to the host molecules having a energy difference (energy barrier) therebetween to form excitons and then transform energy to the dopant molecules. Sequentially, the dopant molecules in the excited state come back to the ground state to emit light. Therefore, the energy level (either singlet or triplet energy structure) of the phosphorescence host material must be higher than that of the phosphorescence dopant material, in which the energy barrier (HOMO-LUMO) of the singlet energy level of the phosphorescence host material is even greater. As to the OLED (such as a blue phosphorescent light-emitting diode) having higher energy barrier, even higher operating voltage is required to convert electric energy to light wave. Further, because the energy barrier of the singlet energy level of the blue phosphorescent host material is particularly large, such that the operating voltage of the blue phosphorescent OLED would be significantly greater than that for the components of other colors.
Therefore, in the conventional photoluminescence device, there is still a need to decrease operating voltage and improve luminous efficiency.